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Chapter 5: Making Decisions Programming Logic and Design, Third Edition Comprehensive.

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1 Chapter 5: Making Decisions Programming Logic and Design, Third Edition Comprehensive

2 2 Objectives After studying Chapter 5, you should be able to: Evaluate Boolean expressions to make comparisons Use the logical comparison operators Understand AND logic Understand OR logic

3 Programming Logic and Design, Third Edition Comprehensive3 Objectives ( continued ) Use selections within ranges Understand precedence when combining AND and OR selections Understand the case structure Use decision tables

4 Programming Logic and Design, Third Edition Comprehensive4 Evaluating Boolean Expressions to Make Comparisons The selection structure (sometimes called a decision structure) is one of the basic structures of structured programming

5 Programming Logic and Design, Third Edition Comprehensive5 Evaluating Boolean Expressions to Make Comparisons ( continued ) A dual-alternative, or binary, selection: –has an action associated with each of two possible outcomes This selection structure is also called an if-then- else structure because it fits the statement: if the answer to the question is yes, then do something else do somethingElse endif

6 Programming Logic and Design, Third Edition Comprehensive6 Evaluating Boolean Expressions to Make Comparisons ( continued ) A single-alternative, or unary, selection –action is required for only one outcome of the question A Boolean expression is one that represents only one of two states, usually expressed as true or false Every decision you make in a computer program involves evaluating a Boolean expression

7 Programming Logic and Design, Third Edition Comprehensive7 Using the Logical Comparison Operators Usually, you can compare only values that are of the same type: –compare numeric values to other numeric values –compare character values to other characters You can ask every programming question by using one of only three types of comparison operators in a Boolean expression

8 Programming Logic and Design, Third Edition Comprehensive8 Using the Logical Comparison Operators ( continued ) For any two values that are the same type, you can decide whether: –The two values are equal –The first value is greater than the second value –The first value is less than the second value In any Boolean expression, the two values used can be either variables or constants Each programming language supports its own set of logical comparison operators, or comparison symbols, that express these Boolean tests

9 Programming Logic and Design, Third Edition Comprehensive9 Using the Logical Comparison Operators ( continued ) Most languages allow you to use the algebraic signs for greater than (>) and less than (<) to make the corresponding comparisons Additionally, COBOL, which is very similar to English, allows you to spell out the comparisons in expressions like dayPastDue is greater than 30? or packageWeight is less than maximumWeightAllowed? RPG uses the two letter abbreviations GT and LT to represent greater than or less than

10 Programming Logic and Design, Third Edition Comprehensive10 Using the Logical Comparison Operators ( continued ) Most programming languages also provide for three additional comparisons For any two values that are the same type, you can decide whether: –The first is greater than or equal to the second. –The first is less than or equal to the second. –The two are not equal. Any logical situation can be expressed using just three types of comparisons: equal, greater than, and less than

11 Programming Logic and Design, Third Edition Comprehensive11 Using the Logical Comparison Operators ( continued ) Comparing two amounts to decide if they are not equal to each other is the most confusing of all the comparisons Using “not equal to” in decisions involves thinking in double negatives, which makes you prone to include logical errors in your program

12 Programming Logic and Design, Third Edition Comprehensive12 Using the Logical Comparison Operators ( continued )

13 Programming Logic and Design, Third Edition Comprehensive13 Using the Logical Comparison Operators ( continued ) Besides being awkward to use, the “not equal to” comparison operator is the one most likely to be different in various programming languages –COBOL allows you to write “not equal to” –Pascal uses a less-than sign followed immediately by a greater-than sign (<>) –C#, C++, C and Java use an exclamation point followed by an equal sign (!=)

14 Programming Logic and Design, Third Edition Comprehensive14 Understanding AND Logic Often, you need more than one selection structure to determine whether an action should take place For example, suppose that your employer wants a report that lists workers who have registered for both insurance plans offered by the company: the medical plan and the dental plan Known as an AND decision because the employee’s record must pass two tests— participation in the medical plan and participation in the dental plan—before you write that employee’s information on the report

15 Programming Logic and Design, Third Edition Comprehensive15 Understanding AND Logic ( continued ) A compound, or AND, decision requires a nested decision, or a nested if—that is, a decision “inside of” another decision

16 Programming Logic and Design, Third Edition Comprehensive16 Writing Nested AND Decisions for Efficiency When you nest decisions because the resulting action requires that two conditions be true, you must decide which of the two decisions to make first Logically, either selection in an AND decision can come first However, when there are two selections, you often can improve your program’s performance by making an appropriate choice as to which selection to make first

17 Programming Logic and Design, Third Edition Comprehensive17 Writing Nested AND Decisions for Efficiency ( continued ) In many AND decisions, you have no idea which of two events is more likely to occur; –In that case, you can legitimately ask either question first In addition, even though you know the probability of each of two conditions, the two events might not be mutually exclusive –In other words, one might depend on the other

18 Programming Logic and Design, Third Edition Comprehensive18 Combining Decisions in an AND Selection Most programming languages allow you to ask two or more questions in a single comparison by using a logical AND operator If the programming language you use allows an AND operator, you still must realize that –the question you place first is the question that will be asked first –cases that are eliminated based on the first question will not proceed to the second question

19 Programming Logic and Design, Third Edition Comprehensive19 Combining Decisions in an AND Selection ( continued ) The computer can ask only one question at a time

20 Programming Logic and Design, Third Edition Comprehensive20 Combining Decisions in an AND Selection ( continued )

21 Programming Logic and Design, Third Edition Comprehensive21 Avoiding Common Errors in an AND Selection When you must satisfy two or more criteria to initiate an event in a program, you must make sure that the second decision is made entirely within the first decision Beginning programmers often make another type of error when they must make two comparisons on the same field while using a logical AND operator

22 Programming Logic and Design, Third Edition Comprehensive22 Understanding OR Logic Sometimes, you want to take action when one or the other of two conditions is true Called an OR decision because –either one condition must be met or –some other condition must be met, in order for an event to take place If someone asks you, “Are you free Friday or Saturday?,” only one of the two conditions has to be true in order for the answer to the whole question to be “yes” –only if the answers to both halves of the question are false is the value of the entire expression false

23 Programming Logic and Design, Third Edition Comprehensive23 Understanding OR Logic ( continued )

24 Programming Logic and Design, Third Edition Comprehensive24 Incorrect Flowchart for mainLoop() This flowchart is not allowed because it is not structured

25 Programming Logic and Design, Third Edition Comprehensive25 Avoiding Common Errors in an OR Selection An additional source of error that is specific to the OR selection stems from a problem with language The way we casually use English can cause an error when a decision based on a value falling within a range of values is required

26 Programming Logic and Design, Third Edition Comprehensive26 Writing OR Decisions for Efficiency You can write a program that creates a report containing all employees who have either the medical or dental insurance by using the mainLoop()

27 Programming Logic and Design, Third Edition Comprehensive27 Writing OR Decisions for Efficiency ( continued )

28 Programming Logic and Design, Third Edition Comprehensive28 Writing OR Decisions for Efficiency ( continued )

29 Programming Logic and Design, Third Edition Comprehensive29 Writing OR Decisions for Efficiency ( continued ) One of these selections is superior to the other Using either scenario, 950 employee records appear on the list, but the logic used in Figure requires 1,100 decisions, whereas the logic used in Figure 5-27 requires 1,500 decisions The general rule is: In an OR decision, first ask the question that is more likely to be true

30 Programming Logic and Design, Third Edition Comprehensive30 Combining Decisions in an OR Selection If you need to take action when either one or the other of two conditions is met, you can use two separate, nested selection structures, as in the previous examples However, most programming languages allow you to ask two or more questions in a single comparison by using a logical OR operator

31 Programming Logic and Design, Third Edition Comprehensive31 Using Selections Within Ranges Business programs often need to make selections based on a variable falling within a range of values When you use a range check, you compare a variable to a series of values between limits To perform a range check, make comparisons using either the lowest or highest value in each range of values you are using

32 Programming Logic and Design, Third Edition Comprehensive32 Common Errors Using Range Checks Two common errors that occur when programmers perform range checks both entail doing more work than is necessary Figure 5-33 shows a range check in which the programmer has asked one question too many Another error that programmers make when writing the logic to perform a range check involves asking unnecessary questions

33 Programming Logic and Design, Third Edition Comprehensive33 Inefficient Range Selection Including Unreachable Path

34 Programming Logic and Design, Third Edition Comprehensive34 Understanding Precedence When Combining AND and OR Selections Most programming languages allow you to combine as many AND and OR operators in an expression as needed The logic becomes more complicated when you combine AND and OR operators within the same statement When you combine AND and OR operators, the AND operators take precedence, meaning their Boolean values are evaluated first You can avoid the confusion of mixing AND and OR decisions by nesting if statements instead of using ANDs and ORs

35 Programming Logic and Design, Third Edition Comprehensive35 Understanding the Case Structure The Logic below is completely structured

36 Programming Logic and Design, Third Edition Comprehensive36 Understanding the Case Structure ( continued ) Writing the logic using a case structure, as shown in Figure 5-37, might make it easier to understand The case structure provides a convenient alternative to using a series of decisions when you must make choices based on the value stored in a single variable When using the case structure, you test a variable against a series of values, taking appropriate action based on the variable’s value

37 Programming Logic and Design, Third Edition Comprehensive37 Flowchart and Pseudocode of Housing Model Using the Case Structure

38 Programming Logic and Design, Third Edition Comprehensive38 Using Decision Tables A decision table is a problem-analysis tool that consists of four parts: –Conditions –Possible combinations of Boolean values for the conditions –Possible actions based on the conditions –The specific action that corresponds to each Boolean value of each condition

39 Programming Logic and Design, Third Edition Comprehensive39 Summary Every decision you make in a computer program involves evaluating a Boolean expression For any two values that are the same type, you can use logical comparison operators to decide whether –the two values are equal, –the first value is greater than the second value, or – the first value is less than the second value

40 Programming Logic and Design, Third Edition Comprehensive40 Summary ( continued ) Most programming languages allow you to ask two or more questions in a single comparison by using a logical AND operator An OR decision occurs when you want to take action when one or the other of two conditions is true

41 Programming Logic and Design, Third Edition Comprehensive41 Summary ( continued ) Most programming languages allow you to ask two or more questions in a single comparison by using a logical OR operator The case structure provides a convenient alternative to using a series of decisions when you must make choices based on the value stored in a single variable


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